Zinc oxide (ZnO), like other transparent conducting oxides, is an important material for-next generation short-wavelength optoelectronic devices such as low cost, optically transparent light-emitting diodes (LEDs) and laser diodes, transparent p-n junctions, large area flat-panel displays, and solar cells. J. Hu and R. Gordon, Solar cells, 30, 437 (1991); A. E. Delahoy and M. Cherny, Mater. Res. Soc. Symp.Proc. 426, 467 (1996); and P. Yu, Z. K. Tang, G. K. L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, and Y. Segawa, Proc. 23th Int. Conf. Physics of Semiconductors, Berlin, 1996, vol. 2, p. 1453.
To obtain these device applications, it is important to provide the technology necessary for fabrication of both high quality p-type and n-type ZnO films. However, like most wide-band-gap semiconductors, ZnO has the asymmetric doping limitations, G. F. Neumark, Phys. Rev. Lett. 62, 1800 (1989), T. Minami, H. Sato, H. 25 Nanto, and S. Tanaka, Jpn. J. Appl. Phys. 24, L781 (1985), S. B. Zhang, S. -H. Wei, and A. Zunger, J. Appl. Phys. 83, 3192 (1998), D. J. Chadi, Phys. Rev. Lett. 72, 534 (1994); S. B. Zhang, S. -H. Wei, and A. Zunger, Phys. Rev. Lett. 84, 1232 (2000), i.e., it can be easily doped high quality n-type, T. Minami, H. Sato, H. Nanto, and S. Tanaka, Jpn. J. Appl. Phys. 24, L781 (1985), ibid. 25, L776 (1986), J. Hu and R. G. Gordon, Solar Cells 30, 437 (1991), but difficult to dope p-type.
Accordingly, fabricating high quality p-type films is the existing challenge in order to obtain the device application of ZnO. Nitrogen, a good p-type dopant for other II-VI semiconductors, R. M. Park, M. B. Troffer, C. M. Rouleau, J. M. DePuydt, and M. A. Haase, Appl. Phys. Lett. 57, 2127 (1990), has long been considered as a possible dopant for p-type ZnO. A. Kobayashi, O. F. Sankey, and J. D. Dow, Phys. Rev. B 28, 946 (1983). Recent theoretical studies have revealed that the difficulty of making p-type ZnO results from the compensation of low formation energy donor like defects, mainly O vacancies (Vo). A. F. Kohan, G. Ceder, D. Morgan and C. G. van de Walle, Phys. Rev. B 61, 15019 (2000), S. B. Zhang, S. -H. Wei, and A. Zunger, Phys. Rev. B 63, 75205 (2001), Y. Yan, S, J. Pennycook, and S. T. Pantelides, (to be published).
Yamamoto and Katayama-Yoshida proposed that high carrier concentration p-type ZnO films can be achieved by Ga and N codoping approach, T. Yamamoto and H. Katayama-Yoshida, Jpn. J. Appl. Phys. 38, L 1 66 (1999), based on their theoretical findings, i.e., the formation of acceptor-donor-acceptor complexes (No—Gazn—No) to significantly reduce the Madelung energy of the system. By applying this approach, Joseph et al. reported that high carrier concentration (4×1019 cm−3), but very low mobility (0.07 cm2/V.s) p-type ZnO films have been observed using N2O gas with electron cyclotron resonance (ECR) plasma and an additional Ga source. M. Joseph, H. Tabata, and T. Kawai, Jpn. J. Appl. Phys. 38, L1205 (1999). However, the codoping approach can only be realized successfully at a very narrow growth window, e.g., low substrate temperature and restrictedly controlled vapor pressure. H. Katayama-Yoshida, MRS Workshop on Transparent Conducting Oxides, (Denver, 2000). These stringent restrictions are due to the complexity of utilizing specific metastable configuration of No—Gzn—No, multidopant sources and plasma. Thus, the application of the codoping approach is limited, and new methods for fabricating high carrier concentration p-type ZnO films are needed.
We are aware of only three reports on p-type ZnO films. However, they used different dopants. In one report, ammonia was used as a dopant (K. Minegishi, Y.Koiwai, Y. Kikuchi, K. Yano, M. Kasuga, and A. Shimizu, Jpn, J. Appl. Phys. 36, L1453 (1997)). In the other two reports, N2O gas was used as a dopant (M. Joseph, J. Tabata, and T. Kawai, Jpn. J. Appl. Phys. 38, L1205 (1999); X. L. Guo, H. Tabata, and T. Kawai, J. Crystal Growth 223, 135 (2001)). Some advantages of our invention have been addressed in our provisional application.
U.S. Pat. Nos. 5,578,501; 5,420,043; and 5,324,365 each disclose methods for manufacturing solar cells in which a ZnO transparent conducting layer is used as electrodes. The ZnO layer used in these three patents are not made to be p-type conductors.
A method for continuously depositing transparent oxide material (including ZnO) by chemical pyrolysis is disclosed in U.S. Pat. No. 5,180,686. This patent only disclose a method for making regular ZnO films for solar cell use, in which the ZnO is an n-type conductor.
U.S. Pat. No. 5,612,229 discloses a method for manufacturing solar cells in which a ZnO transparent conductor layer is used as electrodes. This patent does not disclose how to make p-type ZnO and other metal oxide films.
U.S. Pat. Nos. 5,804,466 and 6,238,808 B1 disclose methods of producing high quality ZnO films for use in solar cells. These patents do not disclose any method for making p-type ZnO films.
Methods for manufacturing solar cells is disclosed in U.S. Pat. Nos. 5,716,480 and 5,913,986; however, these patents do not disclose methods for making p-type ZnO films. U.S. Pat. No. 5,458,753 disclose better quality ZnO films containing Ga. The ZnO films are n-type materials.
A method of producing n-type ZnO film used as window layers in solar cells is disclosed in U.S. Pat. No. 6,040,521.
U.S. Pat. No. 5,990,416 discloses a method to reduce a dopant in metal oxide films. The dopant is a metal element. This patent does not disclose any method for making p-type ZnO films.
U.S. Pat. No. 5,078,803 discloses a method for manufacturing solar cells incorporating transparent electrodes comprising hazy ZnO. This patent does not disclose any method for making p-type ZnO films.
A method for manufacturing a thin film photovoltaic device comprising a transparent conductive film, which may be ZnO is disclosed in U.S. Pat. No. 6,187,150 B1. This patent does not disclose any method for making p-type ZnO films.
U.S. Pat. No. 5,620,924 discloses a method of preventing deterioration of film quality of transparent conductive film, which may be ZnO. This patent does not disclose any method for making p-type ZnO films.
A process for producing a thin film solar cell is disclosed in U.S. Pat. No. 6,242,687 B1. This patent does not disclose any method for making p-type ZnO films.
U.S. Pat. No. 6,107,116 discloses a method for producing a photovoltaic element with ZnO as a layer having increasing F content in the layer thickness direction. This patent does not disclose a method for making p-type ZnO films.
A method for manufacturing photovoltaic devices is disclosed in U.S. Pat. No. 6,043,427. The method does not disclose making p-type ZnO films.
A method for manufacturing photovoltaic devices comprising ZnO films is disclosed in U.S. Pat. No. 5,604,133. This patent does not disclose making p-type ZnO films.
U.S. Pat. No. 4,612,411 discloses a method for producing thin film solar cells with ZnO window layers. This patent does not disclose methods for making p-type ZnO films.